Non-canonical function of DPP4 promotes cognitive impairment through ERp29-associated mitochondrial calcium overload in diabetes

Elevated activity of plasma dipeptidyl peptidase 4 upon stress can be targeted to reverse tumor immunosuppression

The interplay between stress-induced metabolic reprogramming and perturbations in the cancer-immune dialogue is a challenging research topic with huge knowledge gaps to fill. In a repeated social defeat model, we discovered that circulating corticosterone, blood glucose, and plasma DPP4 activity were increased in stressed mice. Consistently, three independent cohort studies showed that plasma DPP4 activity was positively correlated with the severity of psychological distress of newly diagnosed cancer patients. Stress-induced surge of glucocorticoid can boost DPP4 activity via glucocorticoid receptor signaling without influencing Dpp4 transcription or the abundance of soluble DPP4. Albeit catalytic inhibition of DPP4 upon stress can't normalize the behavioral pattern and glucocorticoid secretion, it managed to reverse the expansion of circulating neutrophils and monocytes, restored the efficacy of prophylactic tumor vaccine, and augmented the priming of tumor-antigen specific T cells. DPP4 blockade in the context of stress largely enhanced the intratumoral accumulation of CD8+T cells and DCs, cytokine production by CD8+T and NK cells in situ, and tumor antigen presentation in vitro. Proteome profiling of mouse plasma revealed stress-related DPP4-sensitive changes that can be linked to immunological alterations and disturbed protease network. Altogether, elevated DPP4 activity may be targeted in cancer patients with psychiatric comorbidities to boost anti-tumor immunity.

Trelagliptin Ameliorates Memory Decline in Diabetic Rats through the AMPK/AKT/GSK-3β Pathway in the Cerebral Cortex

Examining how hypoglycemic medications affect brain function is one of the best approaches to addressing cognitive impairment. In this study, trelagliptin, a dipeptidyl peptidase-4 (DPP4) inhibitor, was utilized to assess memory loss in diabetic rats through fear conditioning tests. Trelagliptin restored fear memory in diabetic rats that had been disrupted over a relatively long period (24 h) or extended period (5 days). Moreover, trelagliptin treatment reduced the higher incidence of neuronal cell death in the cerebral cortex, as observed via Nissl or hematoxylin and eosin staining. Subsequent analyses revealed that diabetic rats exhibited elevated levels of inflammatory cytokines (p-IKKα and p-NFκB) and a trend toward oxidative damage, indicated by malondialdehyde (MDA), superoxide dismutase 2 (SOD2), and glutathione peroxidase 4 (GPX4) detection. However, administration of trelagliptin reversed these markers to baseline levels. Additionally, trelagliptin activated p-AMPK, p-AKT, and p-GSK-3β. Notably, trelagliptin upregulated the expression of postsynaptic density protein 95 (PSD95) and synaptotagmin 1 (SYT1) while downregulating amyloid precursor protein (APP) and beta-site amyloid precursor protein cleaving enzyme 1 (BACE1). These findings suggest that trelagliptin alleviates cognitive impairment in diabetic rats, likely through AMPK-AKT-GSK-3β-mediated mitigation of oxidative stress, enhancement of synaptic plasticity, and reduction of Aβ accumulation.

Phosphatidate phosphatase Lipin1 alters mitochondria-associated endoplasmic reticulum membranes (MAMs) homeostasis: effects which contribute to the development of diabetic encephalopathy

Diabetic encephalopathy (DE) is a common, chronic central nervous system complication of diabetes mellitus, and represents a condition without a clear pathogenesis or effective therapy. Findings from recent studies have indicated that a dyshomeostasis of mitochondria-associated endoplasmic reticulum membranes (MAMs) may be involved in the development of neurodegenerative diseases such as DE. MAMs represent a dynamic contact site between mitochondrial and endoplasmic reticulum (ER) membranes, where phospholipid components are exchanged with each other. Previous work within our laboratory has revealed that Lipin1, a critical enzyme related to phospholipid synthesis, is involved in the pathogenesis of DE. Here, we show that Lipin1 is downregulated within the hippocampus of a DE mouse model, an effect which was accompanied with a decrease in MAMs. Knockdown of Lipin1 in the hippocampus of normal mice resulted in a reduction of MAMs, ER stress, abnormal mitochondrial function, as well as impaired synaptic plasticity and cognitive function. These same phenomena were observed in the DE model, while an upregulation of Lipin1 within the hippocampus of DE mice improved these symptoms. Low levels of Lipin1 in DE mice were also associated with neuroinflammation, while an overexpression of Lipin1 significantly ameliorated the neuroinflammation observed in DE mice. In conclusion, Lipin1 ameliorates pathological changes associated with DE in a mouse model via prevention of dyshomeostasis in MAMs. Such findings suggest that Lipin1 may be serve as a new potential target for the treatment of DE.

Endothelial MICU1 protects against vascular inflammation and atherosclerosis by inhibiting mitochondrial calcium uptake

Mitochondrial dysfunction fuels vascular inflammation and atherosclerosis. Mitochondrial calcium uptake 1 (MICU1) maintains mitochondrial Ca2+ homeostasis. However, the role of MICU1 in vascular inflammation and atherosclerosis remains unknown. Here, we report that endothelial MICU1 prevents vascular inflammation and atherosclerosis by maintaining mitochondrial homeostasis. We observed that vascular inflammation was aggravated in endothelial cell-specific Micu1 knockout mice (Micu1ECKO) and attenuated in endothelial cell-specific Micu1 transgenic mice (Micu1ECTg). Furthermore, hypercholesterolemic Micu1ECKO mice also showed accelerated development of atherosclerosis, while Micu1ECTg mice were protected against atherosclerosis. Mechanistically, MICU1 depletion increased mitochondrial Ca2+ influx, thereby decreasing the expression of the mitochondrial deacetylase sirtuin 3 (SIRT3) and the ensuing deacetylation of superoxide dismutase 2 (SOD2), leading to the burst of mitochondrial reactive oxygen species (mROS). Of clinical relevance, we observed decreased MICU1 expression in the endothelial layer covering human atherosclerotic plaques and in human aortic endothelial cells exposed to serum from patients with coronary artery diseases (CAD). Two-sample Wald ratio Mendelian randomization further revealed that increased expression of MICU1 was associated with decreased risk of CAD and coronary artery bypass grafting (CABG). Our findings support MICU1 as an endogenous endothelial resilience factor that protects against vascular inflammation and atherosclerosis by maintaining mitochondrial Ca2+ homeostasis.

O-GlcNAcylation of circadian clock protein Bmal1 impairs cognitive function in diabetic mice

Neuronal damage in the hippocampus induced by high glucose has been shown to promote the onset and development of cognitive impairment in diabetes, but the underlying molecular mechanism remains unclear. Guided by single-cell RNA sequencing, we here report that high glucose increases O-GlcNAcylation of Bmal1 in hippocampal neurons. This glycosylation promotes the binding of Clock to Bmal1, resulting in the expression of transcription factor Bhlhe41 and its target Dnajb4. Upregulated Dnajb4 in turn leads to ubiquitination and degradation of the mitochondrial Na + /Ca2+ exchanger NCLX, thereby inducing mitochondrial calcium overload that causes neuronal damage and cognitive impairment in mice. Notably, Bhlhe41 downregulation or treatment with a short peptide that specifically blocks O-GlcNAcylation of Bmal1 on Ser424 mitigated these adverse effects in diabetic mouse models. These data highlight the crucial role of O-GlcNAcylation in circadian clock gene expression and may facilitate the design of targeted therapies for diabetes-associated cognitive impairment.

Dipeptidyl peptidase-4 inhibitors alleviate cognitive dysfunction in type 2 diabetes mellitus

BACKGROUND

Patients with type 2 diabetes mellitus (T2DM) are commonly at high risk for developing cognitive dysfunction. Antidiabetic agents might be repurposed for targeting cognitive dysfunction in addition to modulation on glucose homeostasis. This study aimed to evaluate the impact of dipeptidyl peptidase-4 inhibitors (DPP-4i) on cognitive function in T2DM.

METHODS

PubMed, Embase, Cochrane Library and Web of Science were systematically searched from inception to September 30, 2023. Weighted mean differences were calculated using the Mantel-Haenszel (M-H) fixed or random effects model based on the degree of heterogeneity among studies. Heterogeneity was evaluated using a Chi-squared test and quantified with Higgins I2. Sensitivity analysis was performed with the leave-one-out method, and publication bias was evaluated according to Begg's and Egger's tests.

RESULTS

Six clinical trials involving 5,178 participants were included in the pooled analysis. Administration of DPP-4i generally correlated with an increase of Mini-Mental State Examination (MMSE) scores (1.09, 95% CI: 0.22 to 1.96). DPP-4i alleviated cognitive impairment in the copying skill subdomain of MMSE (0.26, 95% CI: 0.12 to 0.40). Treatment with DPP-4i also resulted in an increase of Instrumental Activities of Daily Living (IADL) scores (0.82, 95% CI: 0.30 to 1.34). However, DPP-4i produced no significant effects on Barthel Activities of Daily Living (BADL) scores (0.37, 95% CI: -1.26 to 1.99) or other test scores.

CONCLUSIONS

DPP-4i treatment favourably improved cognitive function in patients with T2DM. Further trials with larger samples should be performed to confirm these estimates and investigate the association of different DPP-4i with cognitive function among diabetic patients.

TRIAL REGISTRATION IN PROSPERO

CRD42023430873.